"She reads easily."
Translation:Zij leest gemakkelijk.
They are completely interchangeable. They are variations of the same word, and both have been around for a long time. The only difference is that "gemakkelijk" sounds slightly more formal than "makkelijk". I think people will form their own preferences as to which one they use when, and what sounds better to them, but I can't think of any situation where only one of them would be correct.